1. Field of the Invention
The present invention relates to processes for extracting and purifying the gallium contained in sodium aluminate solutions.
2. Discussion of the Background
Gallium, which has been known for a long time merely for its properties of melting at low temperature allowing the production of low melting point alloys, has benefitted for some years from considerable renewed recognition owing, in particular, to the development of gallium arsenide for the electronics industry. Gallium arsenide is preferred, for example, to silicon as semiconductor in certain very specific conditions of use.
A significant amount of gallium currently originates from Bayer liquors which are sodium aluminate solutions resulting from the attack on bauxite by sodium hydroxide in the Bayer process which is well known for the production of alumina trihydrate. Despite a relatively high content of from 150 to 500 mg of gallium per liter of liquor, it is difficult to recover the gallium selectively in the presence of large quantities of aluminum, which has very similar chemical properties, and of other impurities which are soluble in a highly alkaline medium, vanadate, zincate, ferrate, molybdate, etc.
Electrolysis using a mercury cathode was the only process adopted when the quantities of gallium to extract were small. But, with increasing requirements for gallium, the problems posed by the handling of large quantities of mercury, the low faradic yields and, in particular, the inadequate purity of the product obtained, the person skilled in the art has rapidly turned to processes in which the stages of extraction of dissolved gallium and of reduction to the metallic state by cementation or electrolysis are quite distinct and often separated by intermediate and complementary purification and concentration operations.
The improvement in the quality of the product, on the other hand, has entailed some more highly complex and therefore more cumbersome processes and has, in fact, led to an increase in the sources of material losses and to increasingly awkward operating and production conditions.
However, interesting results have been obtained in the initial stage of recovering gallium from alkaline solutions of sodium aluminate by liquid-liquid extraction processes employing 8-hydroxyquinoline or oxine. In particular, interesting results have been obtained with 8-hydroxyquinolines substituted at the 7-position, according to U.S. Pat. No. 3,637,711, which act as complexing agents for gallium, allowing the use of non-chlorinated solvents.
At the same time, the problem of improving the kinetics of liquid-liquid extraction of the gallium has been investigated by the company Rhone-Poulenc. Thus, significant progress has been made by increasing the exchange surfaces between the extracting agent, 7-alkenyl-8-hydroxyquinoline, and the gallium-containing alkaline solution by the formation of a microemulsion (EP 0102280 and EP 0102882 (=U.S. Pat. No. 4,559,203)).
However, owing to the considerable volumes of extracting agent and of solvent to be used, then to be regenerated in order to extract the gallium contained in the Bayer liquor, on an industrial scale, considerable research has been undertaken in order to extract the gallium on ion exchange resins. Thus, U.S. Pat. No. 4,468,374 has proposed extraction using a resin containing the amide-oxime function as the active grouping. Although apparently more attractive than liquid-liquid extraction, this process has turned out to be very difficult to carry out due to the fragility of the amide-oxime groupings and to the degradation of the resin during the successive cycles of elution in an acid medium of the gallium fixed on the resin.
For its part, the company Mitsubishi Chemical Industry, in its Japanese patent published under No. J60-042234A (=U.S. Pat. No. 4,631,177), has proposed that adsorbent resins based on macroporous polymers be impregnated with complexing agents for the 7-alkenyl-8-hydroxyquinoline group, demonstrating that it was possible to fix small quantities of gallium in solution on the stationary phase thus formed, then to elute the gallium with conventional mineral acids. However, this patent does not mention the capacities of these resins more specifically with regard to the charges of gallium which might be fixed. Furthermore, all the extraction tests described have been carried out on a very dilute sodium aluminate solution, ruling out direct application of the process to industrial Bayer liquors.
Quite recently, in a communication sent to the I.S.E.C. in Munich in September 1986, Cote and Bauer have examined the influence of various parameters, such as the quantity of extracting or complexing agent fixed on the resin, the concentrations of gallium, aluminum and sodium hydroxide, the chemical nature of the adsorbent resin as well as the effect of additives capable of activating the kinetics, on various adsorbent resins sold under the generic name of Amberlite XAD and impregnated with 7-alkyl-8-hydroxyquinoline sold under the trade name of "Kelex 100". It has been found that fixing capacities higher than 3 grams of gallium per liter of resin have been obtained with the Amberlite XAD 7 resin having an acrylic ester skeleton and a semi-polar nature, but with sodium aluminate solutions having a gallium concentration at least 5 times higher than that of industrial liquors, ruling out any possibility of application of the process to these liquors whose concentration cannot be altered in any case.
Finally, a practical drawback inherent in the majority of industrial processes employing porous adsorbent resins is the preparation of the resin, during which the resin is hydrated and degassed under vacuum. This operation becomes very awkward to carry out once several hundreds of liters of resin have to be treated.
Under these circumstances, the only industrial processes by which a person skilled in the art can recover and purify gallium directly from alkaline solutions of sodium aluminate are liquid-liquid extraction processes using, as extracting agent, 8-hydroxyquinolines which are substituted at the 7-position with their known disadvantages of the high investment and production cost resulting from the considerable volumes of reagents and, in particular, of extracting agent and solvent to be used, but also from the losses of reagents by degradation and the losses of gallium by entrainment.
There is thus a need for a process for the efficient extraction of gallium contained in sodium aluminate solutions.